Given an integer n, add a dot (".") as the thousands
separator and return it in string format.
Example 1:
Input: n = 987 Output: "987"
Example 2:
Input: n = 1234 Output: "1.234"
Example 3:
Input: n = 123456789 Output: "123.456.789"
Example 4:
Input: n = 0 Output: "0"
Constraints:
0 <= n < 2^31